Oxide thin film transistor and method of fabricating the same

ABSTRACT

The present disclosure relates to an oxide thin film transistor and a fabricating method thereof. In the oxide thin film transistor, which uses amorphous zinc oxide (ZnO) semiconductor as an active layer, damage to the oxide semiconductor due to dry etching may be minimized by forming source and drain electrodes in a multilayered structure having at least two layers, and improving stability and reliability of a device by employing a dual passivation layer structure, which includes a lower layer for overcoming an oxygen deficiency and an upper layer to minimize effects of an external environment on the multilayered source and drain electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the Korean Patent Application No.10-2013-0054551 filed on May 14, 2013 which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an oxide thin film transistor (TFT)and a fabrication method thereof. More particularly, the presentinvention relates to an oxide TFT having an amorphous zinc oxidesemiconductor as an active layer, and a fabrication method thereof.

Discussion of the Related Art

As the consumers' interest in information displays is growing and thedemand for portable (mobile) information devices is increasing, researchand commercialization of light and thin flat panel displays (FPD), whichsubstitute cathode ray tubes (CRTs), the conventional display devices,have increased. Among FPDs, the liquid crystal display (LCD) is a devicefor displaying images by using optical anisotropy of liquid crystal. LCDdevices exhibit excellent resolution, color display and picture quality,so they are commonly used for notebook computers or desktop monitors,and the like.

The LCD includes a color filter substrate, an array substrate and aliquid crystal layer formed between the color filter substrate and thearray substrate.

An active matrix (AM) driving method commonly used for the LCD is amethod in which liquid crystal molecules in a pixel part are driven byusing amorphous silicon thin film transistors (a-Si TFTs) as switchingelements.

A related art LCD will now be described in detail with reference to FIG.1.

FIG. 1 is an exploded perspective view showing a structure of therelated art LCD device.

As shown in FIG. 1, the LCD includes a color filter substrate 5, anarray substrate 10 and a liquid crystal layer 30 formed between thecolor filter substrate 5 and the array substrate 10.

The color filter substrate 5 includes a color filter (C) including aplurality of sub-color filters 7 that implement red, green and bluecolors, a black matrix 6 for dividing the sub-color filters 7 andblocking light transmission through the liquid crystal layer 30, and atransparent common electrode 8 for applying voltage to the liquidcrystal layer 30.

The array substrate 10 includes gate lines 16 and data lines 17 whichare arranged vertically and horizontally to define a plurality of pixelareas (P), TFTs (T), switching elements, formed at respectiveintersections of the gate lines 16 and the data lines 17, and pixelelectrodes 18 formed on the pixel areas (P).

The color filter substrate 5 and the array substrate 10 are attached ina facing manner by a sealant (not shown) formed at an edge of an imagedisplay region to form a liquid crystal panel, and the attachment of thecolor filter substrates 5 and the array substrate 10 is made by anattachment key formed on the color filter substrate 5 or the arraysubstrate 10.

The foregoing LCD is light and has low power consumption, as such, theLCD receives much attention, but the LCD is a light receiving device,not a light emission device, having a technical limitation inbrightness, a contrast ratio, a viewing angle, and the like. Thus, a newdisplay device that can overcome such shortcomings is being activelydeveloped.

An organic light emitting diode (OLED), one of new flat panel displaydevices, is self-emissive, having a good viewing angle and contrastratio compared with the LCD, and because it does not require abacklight, it can be formed to be lighter and thinner. Also, the OLED isadvantageous in terms of power consumption. Besides, the OLED can bedriven with a low DC voltage and has a fast response speed, and inparticular, the OLED is advantageous in terms of a fabrication cost.

Recently, research for an increase of a size of an OLED display deviceis actively ongoing, and in order to achieve such a large-scale OLEDdisplay device, development of a transistor that can secure constantcurrent characteristics as a driving transistor of an OLED to ensure astable operation and durability is required.

An amorphous silicon thin film transistor (TFT) used for theabove-described LCD may be fabricated in a low temperature process, buthas a very low mobility and fails to satisfy a constant current biascondition. Meanwhile, a polycrystalline silicon TFT has a high mobilityand satisfying constant current bias condition but fails to secureuniform characteristics, making it difficult to have a large area andrequiring a high temperature process.

Thus, an oxide TFT including an active layer formed with oxidesemiconductor is being developed. Here, when the oxide semiconductor isapplied to a conventional bottom gate type TFT, the oxide semiconductoris damaged during etching of source and drain electrodes. This maydenature the oxide semiconductor.

FIG. 2 is a sectional view schematically showing a structure of arelated art oxide thin film transistor (TFT).

As shown in FIG. 2, a related art oxide TFT includes a gate electrode 21and a gate insulating layer 15 formed on a substrate 10, and an activelayer 24 formed on the gate insulating layer 15 and made of an oxidesemiconductor material.

Afterwards, source and drain electrodes 22 and 23 are formed on theactive layer 24. Here, while the source and drain electrodes 22 and 23are deposited and etched, the lower active layer 24 (especially, a backchannel region of a part A) may be damaged to be denatured. This maycause a problem in reliability of a device.

That is, since the active layer made of the oxide semiconductor materialdoes not have selectivity with respect to wet etching during etching ofthe source and drain electrodes, dry etching is generally used.Recently, wet etching with improved selectivity is being attempted butdeterioration of device characteristics is caused due to partialetching, which results from low uniformity.

Upon using the wet etching, a loss or damage of the active layer iscaused due to a property of the oxide semiconductor, which is vulnerableto an etchant. Even when the source and drain electrodes are formedusing the dry etching, the active layer is denatured due toback-sputtering and oxygen deficiency of the oxide semiconductor.

Specifically, when molybdenum (Mo) based metals are applied to form thesource and drain electrodes in consideration of contact resistance withthe oxide semiconductor, it is difficult to develop an etchant withselectivity with respect to the oxide semiconductor which is vulnerableto acidity.

As such, due to environmental sensitivity of the oxide semiconductor anddrastic deterioration of the oxide semiconductor by post-processes uponbeing exposed, an etch stopper structure has to be applied. This may,however, increase the number of processes and masks, causing a decreaseof mass production.

SUMMARY OF THE INVENTION

An object of the present disclosure is to provide an oxide thin filmtransistor (TFT) using an amorphous zinc oxide semiconductor as anactive layer and a fabrication method thereof.

Another object of the present disclosure is to an oxide TFT, capable ofpreventing damage to an active layer, which may be caused whenpatterning source and drain electrodes, without an additional process,and improving stability and reliability of a device by applying a dual(double) passivation layer structure, and a fabricating method thereof.

Additional features and advantages of embodiments of the disclosure willbe set forth in the description which follows, and in part will beapparent from the description, or may be learned by practice ofembodiments of the disclosure. These objectives and other advantages ofthe embodiments of the disclosure will be realized and attained by thestructure particularly pointed out in the written description and claimsthereof as well as the appended drawings.

To achieve the above objects and advantages, the present inventionemploys a thin film transistor, comprising: a gate electrode made of afirst conductive film; a gate insulating layer formed on the gateelectrode; an active layer formed on the gate insulating layer, theactive layer made of an oxide semiconductor having zinc based oxide;source and drain electrodes formed on the active layer; a lowerpassivation layer formed on the source and drain electrodes and on theactive layer disposed between the source and drain electrodes, the lowerpassivation layer made of an insulating layer including oxide; and anupper passivation layer formed on the lower passivation layer, the upperpassivation layer made of an insulating layer having higher density thanthe lower passivation layer.

Further, the present invention may employ a thin film transistor,comprising: an active layer made of an oxide semiconductor having zincbased oxide; source and drain electrodes formed on the active layer; agate insulating layer formed on the active layer disposed between thesource and drain electrodes; a gate electrode which is made of a firstconductive film and is formed on the gate insulating layer; a lowerpassivation layer formed on the source and drain electrodes and on theactive layer disposed between the source and drain electrodes, the lowerpassivation layer made of an insulating layer including oxide; and anupper passivation layer formed on the lower passivation layer, the upperpassivation layer made of an insulating layer having higher density thanthe lower passivation layer.

The oxide thin film transistor and the fabricating method thereofaccording to one exemplary embodiment ensures stable and excellentdevice characteristics because the oxide semiconductor is not damagedduring patterning of source and drain electrodes.

The oxide thin film transistor and a fabricating method thereofaccording to one exemplary embodiment minimizes contact resistance byappropriately selecting a lower layer by virtue of the use ofdual-layered source and drain electrodes, and simplify processes byoxidizing Ti into TiO when the Ti is used to form the lower layer of thesource and drain electrodes and using the oxidized TiO as an in-situprotection layer.

The oxide thin film transistor and a fabricating method thereofaccording to one exemplary embodiment provides an improved stability andreliability of a device by forming a dual passivation layer structure,including a lower layer for overcoming a deficiency and an upper layerfor minimizing external affection, on the dual-layered source and drainelectrodes.

The oxide thin film transistor and a fabricating method thereofaccording to one exemplary embodiment provides an improved deviceperformance by simplifying processes and reducing device size, by virtueof excluding a masking process for patterning an etch stopper.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of embodiments as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification, illustrate embodiments of the disclosure andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 is an exploded perspective view showing a related art liquidcrystal display (LCD) device;

FIG. 2 is a sectional view showing the structure of a related art oxidethin film transistor (TFT);

FIG. 3 is a sectional view showing the structure of an oxide TFTaccording to a first exemplary embodiment of the present disclosure;

FIGS. 4A to 4F are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the first exemplaryembodiment of the present disclosure in FIG. 3;

FIG. 5 is a sectional view showing the structure of an oxide TFTaccording to a second exemplary embodiment of the present disclosure;

FIGS. 6A to 6G are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the second exemplaryembodiment of the present disclosure in FIG. 5;

FIG. 7 is a sectional view schematically showing the structure of anoxide TFT according to a third exemplary embodiment of the presentdisclosure; and

FIGS. 8A to 8G are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the third exemplaryembodiment of the present disclosure in FIG. 5.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

An oxide thin film transistor (TFT) and its fabrication method accordingto exemplary embodiments of the present disclosure will now be describedwith reference to the accompanying drawings, such that those skilled inthe art to which the present disclosure belongs can easily practice it.

Advantages and features of the present disclosure and methods forachieving those will be obviously understood by the following exemplaryembodiments described in detail with reference to the accompanyingdrawings. However, the present disclosure is not to be construed asbeing limited to the exemplary embodiments but can be implemented intovarious forms. The exemplary embodiments of the present disclosure aremerely illustrated to fully describe the present disclosure and providedto help a skilled person in the art to understand the scope of thepresent disclosure. The present disclosure is merely defined by theclaims. The same/like reference symbols or numerals over thespecification refer to the same/like components. Sizes of layers andregions and relative sizes in the drawings are exaggerated for clearunderstanding of the description.

The term “an element or layer located/formed on another element orlayer” may indicate not only an element or layer located on anotherelement or layer but also the element or layer located on the anotherelement or layer with interposing another layer or element therebetween.On the other hand, the term “an element or layer located directly on”refers to not interposing another element or layer therebetween.

Spatially relative terms “below or beneath,” “lower,” “above,” “upper”and the like, as shown in the drawings, may be used for easilydescribing relationship between one element or components and anotherelement or components. The spatially relative terms should be construedas terms including different directions of elements during use oroperation, in addition to directions shown in the drawings. For example,if an element shown in the drawing is turned over, an element which isdescribed as “blow” or “beneath” of another element may be located“above” the another element. Therefore, the illustrative term “below” or“beneath” will cover both “below” and “above.”

Terms used herein are merely illustrative and not to be construed aslimiting the present disclosure. The expression in the singular form inthis specification will cover the expression in the plural form unlessotherwise indicated obviously from the context. Terms “comprising,”“including,” and/or “having” used herein should be understood that theyare intended to indicate an existence or addition of several componentsor several steps, several operations and/or elements disclosed in thespecification.

FIG. 3 is a sectional view schematically showing a structure of an oxideTFT according to a first exemplary embodiment of the present disclosure.This exemplarily illustrates a structure of an oxide TFT which uses anamorphous zinc oxide semiconductor as an active layer.

As shown in FIG. 3, the oxide TFT according to the first exemplaryembodiment of the present disclosure may include a gate electrode 121formed on a predetermined substrate 110, a gate insulating layer 115 aformed on the gate electrode 121, an active layer 124 made of anamorphous zinc oxide semiconductor and formed on the gate insulatinglayer 115 a, and source and drain electrodes 122 and 123 electricallyconnected with predetermined regions of the active layer 124.

Here, the oxide TFT according to the first exemplary embodiment may havethe source and drain electrodes 122 and 123 with a double-layeredstructure. This may prevent damage to the amorphous zinc oxidesemiconductor, namely, the active layer 124, which may be caused duringpatterning of the source and drain electrodes 122 and 123, and tominimize contact resistance between the active layer 124 and the sourceand drain electrodes 122 and 123.

That is, the source and drain electrodes 122 and 123 according to thefirst exemplary embodiment may include second source and drainelectrodes 122 b and 123 b formed on an upper layer and made of a metal,such as copper (Cu), aurum (Au), molybdenum (Mo) or the like, eachhaving low resistivity, irrespective of contact resistance with respectto the amorphous zinc oxide semiconductor, and first source and drainelectrodes 122 a and 123 a formed on a lower layer coming in contactwith the active layer 124 and made of a metal, such as titanium (Ti),titanium (Ti) alloy such as molybdenum titanium (MoTi), molybdenum (Mo)or the like, each having selectivity with respect to the second sourceand drain electrodes 122 b and 123 b during wet etching and low contactresistance with respect to the amorphous zinc oxide semiconductor.

Here, damage to the active layer 124, which may be caused during dryetching of the first source and drain electrodes 122 a and 123 a, may beminimized by minimizing a thickness of a metal required to be processedby dry etching.

Here, in the oxide TFT according to the first exemplary embodiment ofthe present disclosure, for example, the active layer may be formed byusing amorphous zinc oxide (ZnO) semiconductor to satisfy high mobilityand constant current test conditions and secure uniform characteristics,having an advantage of being applicable to a large-scale display.

The zinc oxide (ZnO) is a material which can implement three propertiesof conductivity, semiconductor characteristics, and resistivityaccording to the oxygen content, so the oxide TFT employing theamorphous zinc oxide semiconductor material as the active layer may beapplicable to a large-scale display including an LCD device and an OLEDdevice.

Also, recently, huge interest and activity are concentrated on atransparent electronic circuit, and in this case, since the oxide TFTemploying the amorphous zinc oxide semiconductor material as an activelayer has high mobility and can be fabricated at a low temperature, itcan be used for the transparent electronic circuit.

In particular, in the oxide TFT according to the first exemplaryembodiment of the present disclosure, the active layer may be formed byusing a-IGZO semiconductor obtained by containing heavy metals such asindium (In), gallium (Ga), or the like, in ZnO.

The a-IGZO semiconductor, allowing visible ray to be transmittedtherethrough, is transparent, and the oxide TFT fabricated by using thea-IGZO semiconductor has mobility of 1 to 100 cm²/Vs, exhibiting highmobility compared with an amorphous silicon TFT.

Also, since the a-IGZO semiconductor has a wide band gap, it may be usedfor fabricating a UV light emitting diode (LED) having high colorpurity, a white LED, and other components, and also, since the a-IGZOsemiconductor is processed at a low temperature, it can be used toproduce a light, flexible product.

In addition, since the oxide TFT fabricated by using a-IGZOsemiconductor exhibits uniform characteristics similar to those of theamorphous silicon TFT, it may advantageously have a simple componentstructure like the amorphous silicon thin film transistor and beapplicable to a large-scale display.

In the oxide TFT according to the first exemplary embodiment having thecharacteristics, a carrier concentration of the active layer 124 may beadjusted by controlling a concentration of oxygen within reaction gasduring sputtering. This may allow for adjustment of devicecharacteristics of the TFT. As one example, the active layer 124 may beformed using the a-IGZO semiconductor, which is deposited under acondition of 1 to 10% of oxygen concentration.

As the aforementioned oxide TFT according to the first exemplaryembodiment has the source and drain electrodes 122 and 123 formed in amultilayered structure more than two layers, a material with lowresistivity may be selected as an upper layer metal, irrespective ofcontact resistance with the a-IGZO semiconductor, and the lower layermay be formed thin. This may result in minimization of damages to theoxide semiconductor due to dry etching.

Here, even if the source and drain electrodes 122 and 123 are formed inthe multilayered structure, the damage to the oxide semiconductor maynot be fully prevented. There may still remain possibility of causing adeficiency due to damage to a part of the active layer 124.

Therefore, in the oxide TFT according to the first exemplary embodiment,defective regions may be stabilized through surface treatment and alsodefective layers may be stabilized using an upper passivation layer. Toovercome or restore the defective regions, dual (two) passivation layersmay be formed. A lower passivation layer 115 b may be deposited toovercome an oxygen deficiency and an upper passivation layer 115 c maybe deposited to minimize effects of an external environment.

That is, the lower passivation layer 115 b may be formed of aninsulating film, which contains oxygen (preferably, equal or larger than1E+20 atoms/cm³) and a small quantity of hydrogen (preferably, equal orsmaller than 1E+20 atoms/cm³), for curing a back channel region whichsuffers from oxygen deficiency due to dry etching when patterning thefirst source and drain electrodes 122 a and 123 a. As one example, thelower passivation layer 115 b may include MOx, such as TiOx, TaOx, AlOxor the like, or SiOx. Here, to form a porous insulating layer, adeposition temperature may preferably be below 250° C. For example, thelower passivation layer 115 b may be deposited with a thickness of 10 Åto 100 Å at temperature of 100° C. to 250° C. After deposition of thelower passivation layer 115 b under the conditions, thermal treatmentmay further be carried out at temperature of 200° C. to 300° C.

The upper passivation layer 115 c may be a layer for completely blockingthe TFT from external environments, and implemented as a high densitylayer containing SiO₂.

Hereinafter, description will be given in detail of a method forfabricating the oxide TFT according to the first exemplary embodimenthaving the configuration, with reference to the accompanying drawings.

FIGS. 4A to 4F are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the first exemplaryembodiment of the present disclosure in FIG. 3.

As shown in FIG. 4A, a predetermined gate electrode 121 may be formed ona substrate 110 made of a transparent insulating material.

In this case, amorphous zinc oxide composite semiconductor applied tothe oxide TFT may be deposited at a low temperature, so the substrate110, such as a plastic substrate, soda-line glass, or the like, whichmay be applicable to a low temperature process, may be used. Also, sincethe oxide semiconductor exhibits amorphous characteristics, it may beused for a substrate employed in a large-scale display device.

The gate electrode 121 may be formed by depositing a first conductivefilm on an entire surface of the substrate 110 and selectivelypatterning the first conductive film through a photolithography process(a first masking process).

Here, the first conductive film may be made of a low-resistivity obscureconductive material such as aluminum (Al), Al alloy, tungsten (W),copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti),platinum (Pt), tantalum (Ta), and the like. Also, the first conductivefilm may be made of a transparent conductive material such asindium-tin-oxide (ITO), indium-zinc-oxide (IZO), or the like. Also, thefirst conductive film may have a multilayered structure by stacking twoor more conductive materials.

Next, as shown in FIG. 4B, a gate insulating layer 115 a formed of aninorganic insulating layer such as a silicon nitride film (SiNx), asilicon oxide film (SiO₂), or the like, or a high dielectric oxide filmsuch as hafnium (Hf) oxide or aluminum oxide, may be formed on theentire surface of the substrate 110 with the gate electrode 121 formedthereon.

The gate insulating layer 115 a may be formed by chemical vapordeposition (CVD) or plasma enhanced chemical vapor deposition (PECVD).

An amorphous zinc oxide semiconductor layer made of an amorphous zincoxide semiconductor may be formed on the entire surface of the substrate110 with the gate insulating layer 115 a formed thereon and thenpatterned through a photolithography process (a second masking process)to form an active layer 124 made of the amorphous zinc oxidesemiconductor at an upper side of the gate electrode 121.

Here, the amorphous zinc oxide composite semiconductor, especially, thea-IGZO semiconductor may be formed by sputtering a composite target suchas gallium oxide (Ga₂O₃), indium oxide (In₂O₃) and zinc oxide (ZnO). Inaddition, a chemical deposition such as the CVD or atomic layerdeposition (ALD) may be available. Here, the present disclosure may notbe limited to the a-IGZO semiconductor. The active layer 124 may be madeof an oxide semiconductor such as MaMbMcOd (Ma, Mb, Mc: metal)

Also, the a-IGZO semiconductor may form an amorphous zinc oxidesemiconductor layer using composite oxide targets, which containgallium, indium and zinc in atomic ratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1and the like, respectively.

The oxide TFT according to the first exemplary embodiment may be allowedto adjust a carrier concentration of the active layer 124 by controllinga concentration of oxygen within reaction gas during sputtering forforming the amorphous zinc oxide semiconductor layer. Here, uniformdevice characteristics may be secured under a condition of 1 to 10% ofoxygen concentration.

As shown in FIG. 4C, a second conductive film 120 and a third conductivefilm 130 may be formed sequentially on an entire surface of thesubstrate 110 having the active layer 124 thereon.

The second conductive film 120 may be made of a metal, such as titanium,titanium alloy such as molybdenum titanium, molybdenum, or the like,which has selectivity with respect to the upper second source and drainelectrodes upon wet etching for forming the lower first source and drainelectrodes, and low contact resistance with respect to the amorphouszinc oxide semiconductor. The second conductive film 120 may also have amultilayered structure that at least one another conductive material isstacked on a metal, such as the titanium, molybdenum titanium,molybdenum or the like.

Here, the second conductive film 120 according to the first exemplaryembodiment may have a thickness approximately in the range of 50 to 200Å, while having a thickness approximately in the range of 50 to 300 Åupon using dry etching. Accordingly, the damage to the oxidesemiconductor due to the dry etching may be minimized.

Also, since the third conductive film 130 forms the upper second sourceand drain electrodes, the third conductive film 130 may be made of ametal, such as copper (Cu), aurum (Au), molybdenum (Mo) and the like,each of which has low resistivity, irrespective of contact resistancewith the amorphous zinc oxide semiconductor.

Here, prior to depositing the second conductive film 120 on thesubstrate 110 having the active layer 124 formed thereon, apredetermined surface treatment, such as oxygen plasma treatment, may becarried out. This is to supply extra oxygen to a surface of theamorphous zinc oxide semiconductor because of strong oxidative propertyof the titanium when the titanium is selected as the second conductivefilm 120.

As shown in FIG. 4D, the third conductive layer 130 may be selectivelypatterned through a photolithography process (a third masking process),thereby forming second source and drain electrodes 122 b and 123 b,which are formed of the third conductive film 130, on the secondconductive film 120.

Here, etching of the third conductive film 130 may be wet etching whichis appropriate for large scale and uniformity.

By selectively patterning the lower second conductive film 120, firstsource and drain electrodes 122 a and 123 a, which are formed of thesecond conductive film 120, may be formed. Here, etching of the secondconductive film 120 may be dry etching. As aforementioned, since thesecond conductive film 120 is formed thin approximately in the range of50 to 300 Å, damage to the oxide semiconductor due to the dry etchingmay be minimized.

As shown in FIG. 4E, on the entire surface of the substrate 110 havingthe source and drain electrodes 122 and 123 formed thereon may be formeda lower passivation layer 115 b for curing damage to a back channel ofthe active layer 124, caused due to the dry etching of the secondconductive film 120.

That is, as aforementioned, the lower passivation layer 115 b may beformed of an insulating film, which contains oxygen (preferably, equalor larger than 1E+20 atoms/cm³) and a small quantity of hydrogen(preferably, equal or smaller than 1E+20 atoms/cm³), for curing the backchannel region which suffers from the oxygen deficiency due to the dryetching when patterning the first source and drain electrodes 122 a and123 a. As one example, the insulating film may include MOx (M: metal),such as TiOx, TaOx, AlOx and the like, or SiOx.

Here, to form a porous insulating layer, a deposition temperature maypreferably be below 250° C. As one example, the lower passivation layer115 b may be formed with a thickness of 10 Å to 100 Å at temperature of100° C. to 250° C. After depositing the lower passivation layer 115 bunder the conditions, thermal treatment may further be carried out forthe lower passivation layer 115 b at temperature of 200° C. to 300° C.

As shown in FIG. 4F, an upper passivation layer 115 c may be formed onthe entire surface of the substrate 110 having the lower passivationlayer 115 b thereon, so as to completely block the TFT from externalenvironments.

Here, the upper passivation layer 115 c may be formed of a high densityfilm containing SiO₂.

As such, the oxide TFT according to the first exemplary embodiment mayhave passivation layers 115 b and 115 c in the double-layered structure,so as to overcome or restore defective regions.

In the meantime, when the lower layer of the source and drain electrodesis formed of Ti, it may be oxidized into TiO to be used as an in-situprotection layer. This may have advantages in terms of simplifiedprocesses and minimized contact resistance. This will be described indetail in accordance with a second exemplary embodiment of the presentdisclosure.

FIG. 5 is a sectional view schematically showing a structure of an oxideTFT according to a second exemplary embodiment of the presentdisclosure. An oxide TFT according to a second exemplary embodiment mayinclude the same components as the oxide TFT according to the firstexemplary embodiment, excluding that a lower layer of source and drainelectrodes is formed of Ti and oxidized into TiO to be used as anin-situ protection layer.

As shown in FIG. 5, the oxide TFT according to the second exemplaryembodiment of the present disclosure may include a gate electrode 221formed on a predetermined substrate 210, a gate insulating layer 215 aformed on the gate electrode 221, an active layer 224 made of anamorphous zinc oxide semiconductor and formed on the gate insulatinglayer 215 a, and source and drain electrodes 222 and 223 electricallyconnected with predetermined regions of the active layer 224.

Here, in the oxide TFT according to the second exemplary embodiment ofthe present disclosure, the active layer may be formed by usingamorphous zinc oxide (ZnO) semiconductor, as the same as the oxide TFTaccording to the first exemplary embodiment, to satisfy high mobilityand constant current test conditions and secure uniform characteristics,having an advantage of being applicable to a large-scale display.

In the oxide TFT according to the second exemplary embodiment of thepresent disclosure, the active layer 224 may be formed by using a-IGZOsemiconductor obtained by containing heavy metals such as indium (In),gallium (Ga), or the like, in ZnO.

In the oxide TFT according to the second exemplary embodiment having thecharacteristics, a carrier concentration of the active layer 224 may beadjusted by controlling a concentration of oxygen within reaction gasduring sputtering. This may allow for adjustment of devicecharacteristics of the TFT. As one example, the active layer 224 may beformed using the a-IGZO semiconductor, which is deposited under acondition of 1 to 10% of oxygen concentration.

Here, the oxide TFT according to the first exemplary embodiment mayinclude the source and drain electrodes 222 and 223, which are formed tohave a double-layered structure, to prevent damage to the amorphous zincoxide semiconductor, namely, the active layer 224, which may be causedduring patterning of the source and drain electrodes 222 and 223, andminimize contact resistance between the active layer 224 and the sourceand drain electrodes 222 and 223.

That is, the source and drain electrodes 222 and 223 according to thesecond exemplary embodiment, as same as in the first exemplaryembodiment, may include second source and drain electrodes 222 b and 223b formed on an upper layer and made of a metal, such as copper (Cu),aurum (Au), molybdenum (Mo) or the like, each having low resistivity,irrespective of contact resistance with respect to the amorphous zincoxide semiconductor, and first source and drain electrodes 122 a and 123a formed on a lower layer coming in contact with the active layer 124and made of a metal, such as titanium (Ti), titanium (Ti) alloy such asmolybdenum titanium (MoTi), molybdenum (Mo) or the like, each havingselectivity with respect to the second source and drain electrodes 222 band 223 b during wet etching and low contact resistance with respect tothe amorphous zinc oxide semiconductor.

Since the aforementioned oxide TFT according to the second exemplaryembodiment includes the source and drain electrodes 222 and 223 formedin the multilayered structure more than two layers, a material with lowresistivity may be selected as an upper layer metal, irrespective ofcontact resistance with the a-IGZO semiconductor. When the lower layerof the source and drain electrodes 222 and 223 is formed of Ti, the Timay be oxidized into TiO to be used as an in-situ protection layer 215.

To overcome or restore the defective regions, dual (two) passivationlayers may be deposited. A lower passivation layer 215 b may bedeposited to overcome an oxygen deficiency and an upper passivationlayer 215 c may be deposited to minimize effects from an externalenvironment.

That is, the lower passivation layer 215 b may be formed of aninsulating film, which contains oxygen (preferably, equal or larger than1E+20 atoms/cm³) and a small quantity of hydrogen (preferably, equal orsmaller than 1E+20 atoms/cm³), for curing a back channel region whichsuffers from oxygen deficiency. As one example, the lower passivationlayer 215 b may include MOx, such as TiOx, TaOx, AlOx or the like, orSiOx. Here, to form a porous insulating layer, a deposition temperaturemay preferably be below 250° C. For example, the lower passivation layer215 b may be deposited with a thickness of 10 Å to 100 Å at temperatureof 100° C. to 250° C. After deposition of the lower passivation layer215 b under the conditions, thermal treatment may further be carried outat temperature of 200° C. to 300° C.

The upper passivation layer 215 c may be a layer for completely blockingthe TFT from external environments, and implemented as a high densitylayer containing SiO₂.

Hereinafter, description will be given in detail of a method forfabricating the oxide TFT according to the second exemplary embodimenthaving the configuration, with reference to the accompanying drawings.

FIGS. 6A to 6G are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the second exemplaryembodiment of the present disclosure in FIG. 5

As shown in FIG. 6A, a predetermined gate electrode 221 may be formed ona substrate 210 made of a transparent insulating material.

In this case, amorphous zinc oxide composite semiconductor applied tothe oxide TFT may be deposited at a low temperature, so the substrate110, such as a plastic substrate, soda-line glass, or the like, whichmay be applicable to a low temperature process, may be used. Also, sincethe composite semiconductor exhibits amorphous characteristics, it maybe used for the substrate 210 employed in a large-scale display device.

The gate electrode 221 may be formed by depositing a first conductivefilm on an entire surface of the substrate 210 and selectivelypatterning the first conductive film through a photolithography process(a first masking process).

Here, the first conductive film may be made of a low-resistivity obscureconductive material such as aluminum (Al), Al alloy, tungsten (W),copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti),platinum (Pt), tantalum (Ta), and the like. Also, the first conductivefilm may be made of a transparent conductive material such asindium-tin-oxide (ITO), indium-zinc-oxide (IZO), or the like. The firstconductive film may also have a multilayered structure by stacking twoor more conductive materials.

Next, as shown in FIG. 6B, the gate insulating layer 215 a formed of aninorganic insulating layer such as a silicon nitride film (SiNx), asilicon oxide film (SiO₂), or the like, or a high dielectric oxide filmsuch as hafnium (Hf) oxide or aluminum oxide, may be formed on theentire surface of the substrate 210 with the gate electrode 221 formedthereon.

An amorphous zinc oxide semiconductor layer may be formed on the entiresurface of the substrate 210 with the gate insulating layer 215 a formedthereon and then selectively patterned through a photolithographyprocess (a second masking process) to form an active layer 224 made ofthe amorphous zinc oxide semiconductor at an upper side of the gateelectrode 221.

Here, the amorphous zinc oxide composite semiconductor, especially, thea-IGZO semiconductor may be formed by sputtering a composite target suchas gallium oxide (Ga₂O₃), indium oxide (In₂O₃) and zinc oxide (ZnO). Inaddition, a chemical deposition such as the CVD or atomic layerdeposition (ALD) may be available. Here, the present disclosure may notbe limited to the oxide composite semiconductor. The active layer 224may be made of an oxide semiconductor such as MaMbMcOd(Ma, Mb, Mc:metal)

Also, the a-IGZO semiconductor may form the amorphous zinc oxidesemiconductor layer using composite oxide targets, which containgallium, indium and zinc in atomic ratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1and the like, respectively.

The oxide TFT according to the second exemplary embodiment may beallowed to adjust a carrier concentration of the active layer 224 bycontrolling a concentration of oxygen within reaction gas duringsputtering for forming the amorphous zinc oxide semiconductor layer.Here, uniform device characteristics may be secured under a condition of1 to 10% of oxygen concentration.

As shown in FIG. 6C, a second conductive film 220 and a third conductivefilm 230 may be formed on the entire surface of the substrate 210 havingthe active layer 224 thereon.

The second conductive film 220 may be made of titanium, which hasselectivity with respect to upper second source and drain electrodesupon wet etching for forming lower first source and drain electrodes,and low contact resistance with respect to the amorphous zinc oxidesemiconductor. The second conductive film 220 may also have amultilayered structure that at least one another conductive material isstacked on the titanium.

Here, the second conductive film 220 according to the first exemplaryembodiment may have a thickness approximately in the range of 50 to 200Å.

Also, since the third conductive film 230 forms the upper second sourceand drain electrodes, the third conductive film 230 may use a metal,such as copper (Cu), aurum (Au), molybdenum (Mo) and the like, each ofwhich has a low resistivity irrespective of contact resistance withrespect to the amorphous zinc oxide semiconductor.

Here, prior to depositing the second conductive film 220 on thesubstrate 210 having the active layer 224 formed thereon, apredetermined surface treatment, such as oxygen plasma treatment, may becarried out. This is to supply extra oxygen to a surface of theamorphous zinc oxide semiconductor because of strong oxidative propertyof the titanium when the titanium is selected as the second conductivefilm 220.

As shown in FIG. 6D, the third conductive layer 230 may be selectivelypatterned through a photolithography process (third masking process),thereby forming second source and drain electrodes 222 b and 223 b,which are formed of the third conductive film 230, on the secondconductive film 220.

Here, etching of the third conductive film 230 may be wet etching whichis appropriate for large scale and uniformity.

As shown in FIG. 6E, when titanium is employed as the second conductivefilm 220, the exposed second conductive film 220 may be oxidized intoTiO through oxygen plasma treatment or a predetermined thermal treatmentunder oxygen-contained atmosphere after wet etching of the thirdconductive film 230, forming an in-situ protection layer 215 made of theTiO.

Here, the in-situ protection layer 215 located on the active layer 224may protect a back channel of the active layer 224. The secondconductive film 220 on which the second source and drain electrodes 222b and 223 b are located may form the first source and drain electrodes222 a and 223 a, separate from the in-situ protection layer 215.

As such, when the lower layer of the source and drain electrodes 222 and223 is formed of the titanium, the titanium may be oxidized into the TiOto be used as the in-situ protection layer 215. This may result insimplification of processes and minimization of contact resistance.

That is, when the titanium is used as the second conductive film 220, adegree of the titanium being oxidized at room temperature may be ΔH=−940KJ/mol, which is 2.5 times greater than a degree (˜350 KJ/mol) of zincbeing oxidized at room temperature. Therefore, the amorphous zinc oxidesemiconductor may transit into a conductor on the source and drainregions of the active layer 224, which are brought into contact with thefirst source and drain electrodes 222 a and 223 a. This may minimize thecontact resistance and thus improve a device performance.

As shown in FIG. 6F, on the entire surface of the substrate 210 havingthe source and drain electrodes 222 and 223 thereon may be formed alower passivation layer 215 b for curing damage to a back channel of theactive layer 224.

That is, as aforementioned, the lower passivation layer 215 b may beformed of an insulating film, which contains oxygen (preferably, equalor larger than 1E+20 atoms/cm³)) and a small quantity of hydrogen(preferably, equal or smaller than 1E+20 atoms/cm³)), for curing theback channel region which suffers from the oxygen deficiency. As oneexample, the lower passivation layer 215 b may be made of MOx (M:metal), such as TiOx, TaOx, AlOx and the like, or SiOx.

Here, to form a porous insulating layer, a deposition temperature maypreferably be below 250° C. As one example, the lower passivation layer215 b may be deposited with a thickness of 10 Å to 100 Å at temperatureof 100° C. to 250° C. After depositing the lower passivation layer 215 bunder the conditions, thermal treatment may further be carried out attemperature of 200° C. to 300° C.

As shown in FIG. 6G, an upper passivation layer 215 c may be formed onthe entire surface of the substrate 210 having the lower passivationlayer 215 b thereon, so as to completely block the TFT from externalenvironments.

Here, the upper passivation layer 215 c may be formed of a high densityfilm containing SiO₂.

Meanwhile, the dual-layered passivation structure may be applicable to acoplanar TFT as well as a bottom gate type TFT. This will be describedin detail in accordance with a third exemplary embodiment of the presentdisclosure.

FIG. 7 is a sectional view schematically showing a structure of an oxideTFT according to a third exemplary embodiment of the present disclosure,which illustrates an oxide TFT having a self-aligned coplanar structure.

As shown in FIG. 7, the oxide TFT according to the third exemplaryembodiment may include a buffer layer 311 on a predetermined substrate310, an active layer 324 formed on the buffer layer 311 and made ofamorphous zinc oxide semiconductor, a gate electrode 321 formed on theactive layer 324 with interposing a gate insulating layer 311 atherebetween, and source and drain electrodes 322 and 323 formed on theactive layer 324 and electrically connected with predetermined regions,namely, source and drain regions 324 a and 324 b, of the active layer324.

Here, the reference numeral 324 c denotes a channel region, which isdefined by the upper gate electrode 321 to form a conductive channelbetween the source region 324 a and the drain region 324 b.

Here, in the oxide TFT according to the third exemplary embodiment ofthe present disclosure, the active layer 324 may be formed by usingamorphous zinc oxide (ZnO) semiconductor, as the same as the oxide TFTsaccording to the first and second exemplary embodiments, to satisfy highmobility and constant current test conditions and secure uniformcharacteristics, having an advantage of being applicable to alarge-scale display.

In the oxide TFT according to the third exemplary embodiment of thepresent disclosure, the active layer 324 may be formed by using a-IGZOsemiconductor obtained by containing heavy metals such as indium (In),gallium (Ga), or the like, in ZnO.

In the oxide TFT according to the third exemplary embodiment having thecharacteristics, a carrier concentration of the active layer 324 may beadjusted by controlling a concentration of oxygen within reaction gasduring sputtering. This may allow for adjustment of devicecharacteristics of the TFT. As one example, the active layer 324 may beformed using the a-IGZO semiconductor, which is deposited under acondition of 1 to 10% of oxygen concentration.

Since the oxide TFT according to the third exemplary embodiment mayemploy the coplanar structure that the gate electrode 321 and the sourceand drain electrode 322 and 323 are located on the active layer 324,damage to the channel region 324 c of the active layer 324, which may becaused upon etching the source and drain electrodes 322 and 323, may beprevented so as to secure excellent device characteristics.

Here, the coplanar oxide TFT may have offset regions, defined as contactregions between the source and drain regions 324 a and 324 b of theactive layer 324 and the source and drain electrodes 322 and 323 arespaced apart from the gate electrode 321 by predetermined distances. Inthis case, since the offset regions act as resistance regions withhigher resistance than the contact region, unstable devicecharacteristic may be exhibited.

The oxide TFT according to the third exemplary embodiment may employ thecoplanar structure and over-etch the active layer 324 through dryetching during patterning of the gate electrode 321, to make the sourceand drain regions 324 a and 324 b, including the offset regions,conductive, thereby implementing stable device characteristics. Here,the present disclosure may not be limited to this. Making thecorresponding regions conductive may be implemented by remaining(leaving) a partial portion of the gate insulating layer 315 a and thenremoving the remaining (left) gate insulating layer 315 a duringpatterning of the source and drain electrodes 322 and 323. This may havean advantage of preventing a risk that a region, which has been madeconductive in response to the complete removal of the gate insulatinglayer 315 a, is made conductive once again upon patterning the sourceand drain electrodes 322 and 323.

The oxide TFT according to the third exemplary embodiment may includethe source and drain electrodes 322 and 323 having the double-layeredstructure so as to minimize contact resistance between the active layer324 and the source and drain electrodes 322 and 323.

That is, the source and drain electrodes 322 and 323 according to thethird exemplary embodiment, as the same as in the first and secondexemplary embodiments, may include second source and drain electrodes322 b and 323 b formed on an upper layer and made of a metal, such ascopper (Cu), aurum (Au), molybdenum (Mo) or the like, each having lowresistivity, irrespective of contact resistance with respect to theamorphous zinc oxide semiconductor, and first source and drainelectrodes 322 a and 323 a formed on a lower layer coming in contactwith the active layer 324 and made of a metal, such as titanium (Ti),titanium (Ti) alloy such as molybdenum titanium (MoTi), molybdenum (Mo)or the like, each having selectivity with respect to the second sourceand drain electrodes 322 b and 323 b during wet etching and low contactresistance with respect to the amorphous zinc oxide semiconductor.

To overcome or restore the defective regions, dual (two) passivationlayers may be deposited. A lower passivation layer 315 b may bedeposited to overcome a deficiency and an upper passivation layer 315 cmay be deposited to minimize external affection.

Here, the back channel region of the coplanar type oxide TFT accordingto the third exemplary embodiment may be protected by the gateinsulating layer 315 a, but the lower passivation layer 315 b, similarto the first and second exemplary embodiments, may be formed of aninsulating film, which contains oxygen (preferably, equal or larger than1E+20 atoms/cm³)) and a small quantity of hydrogen (preferably, equal orsmaller than 1E+20 atoms/cm³)), for curing the back channel region. Asone example, the lower passivation layer 315 b may include MOx, such asTiOx, TaOx, AlOx or the like, or SiOx. Here, to form a porous insulatinglayer, a deposition temperature may preferably be below 250° C. Forexample, the lower passivation layer 315 b may be deposited with athickness of 10 Å to 100 Å at temperature of 100° C. to 250° C. Afterdeposition of the lower passivation layer 315 b under the conditions,thermal treatment may further be carried out at temperature of 200° C.to 300° C.

The upper passivation layer 315 c may be a layer for completely blockingthe TFT from external environments, and implemented as a high densitylayer containing SiO₂.

Hereinafter, description will be given in detail of a method forfabricating the oxide TFT according to the third exemplary embodimenthaving the configuration, with reference to the accompanying drawings.

FIGS. 8A to 8G are sectional views sequentially showing a fabricationprocess of the oxide TFT illustrated according to the third exemplaryembodiment of the present disclosure in FIG. 7.

As shown in FIG. 8A, a predetermined oxide semiconductor, for example,amorphous zinc oxide semiconductor may be deposited on a substrate 310made of a transparent insulating material, and then selectivelypatterned through a photolithography process (a first masking process),to form an active layer 324, which is made of the oxide semiconductor,on the substrate 310.

Prior to depositing the oxide semiconductor on the substrate 310, abuffer layer 311 may be formed on the substrate 310.

Here, the buffer layer 311 may serve to prevent permeation ofimpurities, such as sodium, existing within the substrate 310 or thelike, into an upper layer during a process like thermal treatment. Inthe present disclosure, since the active layer 324 is formed using theoxide semiconductor, the buffer layer 311 may also be removed.

In this case, oxide semiconductor applied to the oxide TFT may bedeposited at a low temperature, so the substrate 110, such as a plasticsubstrate, soda-line glass, or the like, which may be applicable to alow temperature process, may be used. Also, since the oxidesemiconductor exhibits amorphous characteristics, it may be used for thesubstrate 110 employed in a large-scale display device.

Here, the amorphous zinc oxide composite semiconductor, especially, thea-IGZO semiconductor may be formed by sputtering a composite target,such as gallium oxide (Ga₂O₃), indium oxide (In₂O₃) and zinc oxide(ZnO). In addition, a chemical deposition, such as the CVD or atomiclayer deposition (ALD), may be available. Here, the present disclosuremay not be limited to the oxide composite semiconductor. The activelayer 324 may be made of an oxide semiconductor such as MaMbMcOd(Ma, Mb,Mc: metal).

Also, the a-IGZO semiconductor may form an amorphous zinc oxidesemiconductor layer using composite oxide targets, which containgallium, indium and zinc in atomic ratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1and the like, respectively.

The oxide TFT according to the third exemplary embodiment may be allowedto adjust a carrier concentration of the active layer 324 by controllinga concentration of oxygen within reaction gas during sputtering. Here,uniform device characteristics may be secured under a condition of 1 to10% of oxygen concentration.

As shown in FIG. 8B, a predetermined insulating film and a firstconductive film may be deposited on the substrate 310 with the activelayer 324 formed thereon, and then selectively patterned through aphotolithography process (a second masking process), to form a gateelectrode 321, made of the first conductive film, on the active layer324.

The gate electrode 321 may be formed on the active layer 324 withinterposing therebetween a gate insulating layer 315 a, made of theinsulating film. Here, the active layer 324 and the gate electrode 321may also be formed through one-time masking process by way of using adiffraction mask or a halftone mask.

Here, the insulating film may be an inorganic insulating film such assilicon oxide film, or the like, or a high dielectric oxide film such ashafnium (Hf) oxide or aluminum oxide, and dry etching such as oxygenplasma treatment may be employed. In addition, when the insulating filmis formed of an oxide such as SiOx, HfOx or AlOx, surface treatment orthermal treatment may first be carried out prior to deposition of theinsulating film.

Also, the first conductive film may be made of a low-resistivity obscureconductive material such as aluminum (Al), Al alloy, tungsten (W),copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti),platinum (Pt), tantalum (Ta), and the like. Also, the first conductivefilm may be made of a transparent conductive material such asindium-tin-oxide (ITO), indium-zinc-oxide (IZO), or the like. Also, thefirst conductive film may have a multilayered structure by stacking twoor more conductive materials.

Such dry etching may be carried out to over-etch an exposed region ofthe active layer 324, forming the source and drain regions 324 a and 324b including offset regions.

That is, the active layer 324, which has been exposed when over-etchingthe insulating film through the oxygen plasma treatment for patterningthe gate insulating layer 315 a, may have a decreased resistance by theoxygen plasma. Accordingly, the source and drain regions 324 a and 324 bmay be formed on the active layer 324. Here, the present disclosure maynot be limited to this. The source and drain regions 324 a and 324 b mayalso be formed by changing the resistance of the active layer 324exposed by the surface treatment or thermal treatment, such as theoxygen plasma, after patterning the gate insulating layer 315 a. Also,the source and drain regions 324 a and 324 b may be made conductive byremaining a partial portion of the gate insulating layer 315 a and thenremoving the remaining gate insulating layer 315 a during patterning ofthe source and drain electrodes 322 and 323.

Here, the active layer 324 beneath the gate insulating layer 315 a maydefine a channel region 324 c which forms a conductive channel.

As shown in FIG. 8C, a second conductive film 320 and a third conductivefilm 330 may be formed sequentially on the entire surface with theactive layer 324 formed thereon.

The second conductive film 320 may use a metal, such as titanium,titanium alloy such as molybdenum titanium, molybdenum or the like,which has selectivity with respect to the upper second source and drainelectrodes upon wet etching for forming the first source and drainelectrodes of the lower layer, and low contact resistance with respectto the amorphous zinc oxide semiconductor. The second conductive film320 may have a multilayered structure that at least one anotherconductive material is stacked on a metal, such as the titanium,molybdenum titanium, molybdenum or the like.

Here, the second conductive film 320 according to the third exemplaryembodiment may have a thickness approximately in the range of 50 to 200Å, while having a thickness approximately in the range of 50 to 300 Åupon using dry etching. Accordingly, the damage to the oxidesemiconductor due to the dry etching may be minimized.

Also, since the third conductive film 330 forms the upper second sourceand drain electrodes, the third conductive film 330 may be made of ametal, such as copper (Cu), aurum (Au), molybdenum (Mo) and the like,each of which has a low resistivity, irrespective of contact resistancewith the amorphous zinc oxide semiconductor.

Here, prior to depositing the second conductive film 320 on thesubstrate 310 having the active layer 324 formed thereon, apredetermined surface treatment, such as oxygen plasma treatment, may becarried out. This is to supply extra oxygen to a surface of theamorphous zinc oxide semiconductor because of strong oxidative propertyof the titanium when the titanium is selected as the second conductivefilm 320.

As shown in FIG. 8D, the third conductive layer 330 may be selectivelypatterned through a photolithography process (a third masking process),thereby forming second source and drain electrodes 322 b and 323 b,which are formed of the third conductive film 330, on the secondconductive film 320.

Here, etching of the third conductive film 330 may be wet etching whichis appropriate for large scale and uniformity.

By selectively patterning the lower second conductive film 320, thefirst source and drain electrodes 322 a and 323 a, which are formed ofthe second conductive film 320, may be formed. Here, etching of thesecond conductive film 320 may be dry etching. As aforementioned, sincethe second conductive film 320 is formed thin approximately in the rangeof 50 to 300 Å, damage to the oxide semiconductor due to the dry etchingmay be minimized.

As shown in FIG. 8F, a lower passivation layer 315 b may be formed onthe entire surface of the substrate 310 with the source and drainelectrodes 322 and 323 formed thereon.

That is, the lower passivation layer 315 b may be made of an insulatingfilm containing oxygen and a small quantity of hydrogen. As one example,the lower passivation layer 315 b may be made of MOx (M: metal), such asTiOx, TaOx, AlOx and the like, or SiOx.

Here, to form a porous insulating layer, a deposition temperature maypreferably be below 250° C. For example, the lower passivation layer 315b may be deposited with a thickness of 10 Å to 100 Å at temperature of100° C. to 250° C. After deposition of the lower passivation layer 315 bunder the conditions, thermal treatment may further be carried out attemperature of 200° C. to 300° C.

Alternatively, when titanium is employed as the second conductive film320, the exposed second conductive film 320 may be oxidized into TiOthrough oxygen plasma treatment or a predetermined thermal treatmentunder oxygen-contained atmosphere after wet etching of the thirdconductive film 330, forming an in-situ protection layer 315 made of theTiO.

As shown in FIG. 8G, an upper passivation layer 315 c may be formed onthe entire surface of the substrate 310 having the lower passivationlayer 315 b thereon, so as to completely block the TFT from externalenvironments.

Here, the upper passivation layer 315 c may be formed of a high densityfilm containing SiO₂.

As described above, the present disclosure may be applicable to otherdisplay devices, which are fabricated using TFTs, in addition to the LCDdevice. For example, the present disclosure may be applied even to anOLED display device connected with OLEDs.

Also, the present disclosure may have advantages of being used fortransparent electronic circuits or flexible displays, owing to employingan amorphous zinc oxide semiconductor material, which has high mobilityand can be processed by a low temperature process.

As the present disclosure may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalents of such metes and bounds are therefore intendedto be embraced by the appended claims.

What is claimed is:
 1. A thin film transistor, comprising: a gateelectrode on a substrate; a gate insulating layer on the gate electrode;an active layer on the gate insulating layer, the active layer made ofan oxide semiconductor; a lower conductive layer on the active layer,wherein the lower conductive layer comprises a source electrode, a drainelectrode, and wherein first and second portions of the lower conductivelayer are oxidized and converted to first and second portions of anin-situ protection layer, so that the first portion of the in-situprotection layer is directly on a back-channel region of the activelayer, and the second portion of the in-situ protection layer is at aside of the source electrode or the drain electrode and is directly onand contacting the gate insulating layer; an upper conductive layerdirectly on the lower conductive layer; and a lower passivation layerdirectly on the in-situ protection layer and the upper conductive layer.2. The thin film transistor of claim 1, wherein the lower conductivelayer covers a side portion of the active layer.
 3. The thin filmtransistor of claim 1, wherein the in-situ protection layer includesTiOx (where x=2±∈, where ∈ is sufficiently small to maintainsemiconductor conductivity in TiOx) and the lower conductive layerincludes Ti.
 4. A method of manufacturing a thin film transistorcomprising: forming a gate electrode on a substrate; forming a gateinsulating layer on the gate electrode; forming an active layer on thegate insulating layer, the active layer made of an oxide semiconductor;forming a lower conductive layer on the active layer, wherein the lowerconductive layer comprises a source electrode, a drain electrode, afirst portion, and a second portion; oxidizing the first and secondportions of the lower conductive layer to convert first and secondportions of an in-situ protection layer, so that the first portion ofthe in-situ protection layer is formed directly on a back-channel regionof the active layer, and the second portion of the in-situ protectionlayer is formed at a side of the source electrode or the drain electrodeand is directly formed on and contacting the gate insulating layer;forming an upper conductive layer directly on the lower conductivelayer; and forming a lower passivation layer directly on the in-situprotection layer and the upper conductive layer.
 5. The method of claim4, further comprising forming an upper passivation layer on the lowerpassivation layer.
 6. The method of claim 4, wherein the lowerpassivation layer is formed of a porous metal oxide insulating layer. 7.The thin film transistor of claim 1, further comprising an upperpassivation layer on the lower passivation layer.
 8. The thin filmtransistor of claim 1, wherein the lower passivation layer containsoxygen equal to or larger than 1E+20 atoms/cm³, and hydrogen equal to orsmaller than 1E+20 atoms/cm³ for curing oxygen deficiency at a backchannel region of the active layer.
 9. The thin film transistor of claim1, wherein the lower passivation layer is a porous metal oxideinsulating layer.